Cook the pasta in salted water until al dente. In the meantime, cut the onion into small cubes, heat the olive oil in a pan and sauté the onion until translucent. Add the spinach leaves and let cook. Add the tomatoes, season everything with salt and pepper and simmer for a few minutes. Cut the mozzarella into cubes and fold into the sauce. Drain the pasta, place on a plate and pour the sauce over it.
Per serving: 540 kcal, 85 g carbs, 10 g fat, 17% fat calories